1. Field of the Invention
The present invention relates generally to a MOS transistor process, and more specifically to a MOS transistor process, that forms a recess and/or performs a cleaning process after a source/drain is formed and before an epitaxial structure is formed.
2. Description of the Prior Art
For decades, chip manufacturers have made metal-oxide-semiconductor (MOS) transistors faster by making them smaller. As the semiconductor processes advance to very deep sub micron era such as 65-nm node or beyond, how to increase the driving current of MOS transistors has become a critical issue. In order to improve device performances, crystal strain technology has been developed. Crystal strain technology is becoming more and more attractive as a mean for reaching better performances in the field of CMOS transistor fabrication. Putting a strain on a semiconductor crystal alters the speed at which charges move through that crystal. Strain makes CMOS transistors work better by enabling electrical charges, such as electrons, to pass more easily through the silicon lattice of the gate channel.
In the known arts, attempts have been made to use a strained silicon layer, which has been grown epitaxially on a silicon substrate with an epitaxial structure such as a silicon germanium (SiGe) structure or a silicon carbide (SiC) structure disposed in between. In this type of MOS transistor, a biaxial compressive or tensile strain occurs in the epitaxial structure due to the silicon germanium or the silicon carbide which has a less or larger lattice constant than silicon, and, as a result, the band structure is altered, and the carrier mobility increases. This enhances the speed performance of the MOS transistors.
However, ingredients in the epitaxial structure such as germanium or carbon etc would diffuse outwards when induced by high temperature or ion implantation etc, thereby decreasing the concentration of the ingredients in the epitaxial structure, and degrading the performance of the epitaxial structure. Moreover, the qualities of the surface of a substrate having the epitaxial structure formed thereon will also affect the shape or the cross-sectional profile etc. of the epitaxial structure, which would affect the performances of formed semiconductor component.